1. Field of the Invention
The present invention relates to an aberration correction element, and an optical pickup including the aberration correction element to correct chromatic aberration caused by variation of power from a light source when changing between operating modes in a high-density recording medium, and which is compatible with high-density recording media and relatively low density digital versatile disc (DVD) family media.
2. Description of the Related Art
In optical recording/reproduction systems, recording density is determined by the size of a focused spot (S). In general, the size of the focused spot (S) is proportional to the wavelength (λ), and inversely proportional to the numerical aperture (NA).
To reduce the size of the focused spot, high-density optical pickups adopt a light source emitting blue light, and an objective lens having at least 0.6 NA; thus, a higher information recording density is achieved compared to existing compact discs (CD) and DVD family recording media.
A refractive index of optical materials used to manufacture an objective lens for optical pickups, such as glass and plastic, varies with a wavelength shorter than 650 nm. For this reason, when the wavelength of an incident light beam slightly varies within the short wavelength range, a considerable amount of chromatic aberration occurs.
Optical power of the optical recording/reproduction systems varies according to an operating mode, i.e., a recording mode and a reproduction mode. Accordingly, the wavelength of a light beam emitted from the light source varies within a range of about ±2 nm according to the operating mode. As the output power of the light source is increased to record information, the wavelength of light from the light source becomes longer. As for the optical pickup in high-density recording/reproduction systems, which employs the blue light source and the objective lens optimized for a specific wavelength, a large amount of chromatic aberration is caused by a wavelength variation when the operating mode changes from a recording mode to a reproduction mode, and vice versa.
Such chromatic aberration can be corrected by defocusing the objective lens. However, it takes a relatively long period of time to drive the objective lens using an actuator to follow the wavelength variation. Thus, as the change in operating mode is repeated, the quality of recording and reproduction signals during the period of time it takes to drive the objective lens to follow the wavelength variation deteriorates.
Thus, the optical pickup for high-density recording/reproduction systems needs an optical element capable of effectively reducing chromatic aberration caused by the change in wavelength, which occurs when changing the operating mode. In addition, the compatibility with existing DVDs needs to be considered for the high-density optical pickup. However, for DVD-recordable (DVD-R) and multi-layered DVDs, a red light source is used as the light source because of the notable low reflectivity of these types of disks with respect to short wavelength light.
FIG. 1 illustrates a conventional optical pickup including a refracting and diffracting objective lens 9. The refracting and diffracting objective lens 9 focuses light reflected from a beam splitter 5, after having been emitted from a semiconductor laser 1 and collimated by a collimating lens 3, on the recording surface of an optical disc 10. The refracting and diffracting objective lens 9 is an aspherical lens, which has aspherical light receiving and emitting surfaces and diffraction patterns on the aspherical surfaces. The refracting and diffracting objective lens 9 is constructed as a combination of the refracting and diffracting lenses.
Assuming that V is Abbe's number of refracting lens part and VHOE is Abbe's number of diffracting lens part. Further, n1, n2 and n3 are the refractive indices of the refracting and diffracting objective lens 9 with respect to the central, shortest and longest wavelengths λ1, λ2 and λ3 of light emitted from the semiconductor laser 1, respectively. The refracting and diffracting objective lens 9 is designed such that it satisfies the following condition:(1+VHOE/V)(n2−1)>0.572. where V=(n2−1)/(n1−n3) and VHOE=λ2(λ1−λ3). The refracting and diffracting objective lens 9 has an NA of 0.7 or more, and can eliminate chromatic aberration caused by the wavelength variation emitted from the semiconductor laser 1.
Further, a first skew plate 7a and a second skew plate 7b correct coma aberration caused by skewing of the optical disk 10 in the reproduction mode. The first and the second skew plates 7a and 7b are arranged such that their convex and concave surfaces face outward. As the first and the second skew plates 7a and 7b are moved in opposite directions, the shape of the wavefront changes into the inverse shape of the coma aberration, thereby canceling out the coma aberration caused by the skewing of the optical disc 10. The optical pickup also includes a quarter-wave plate 6, and a photodetector 13.
In terms of the properties of diffraction elements, the optical pickup including the refracting and diffracting objective lens 9 has a low light efficiency which makes it difficult to obtain high optical power sufficient to record information. The conventional refracting and diffracting objective lens 9 has a low light efficiency with respect to the wavelength on the light beam, which is largely deviated from the initial wavelength for the objective lens. Thus, the conventional refracting and diffracting objective lens 9 cannot be adopted for an optical pickup using different wavelengths of light.